This invention relates to a process for the polymerization of olefins employing a metallocene catalyst and to a cocatalyst for activating a metallocene procatalyst to provide the metallocene catalyst.
The most common polyolefin elastomers produced today are copolymers of ethylene and propylene (EP) and terpolymers of ethylene, propylene and a diene (EPDM). Ordinary EP elastomers can be cured using such curatives as organic peroxides, while the use of sulfur as a curative requires the incorporation of a diene. EPDM elastomers are usually produced with vanadium-organoaluminum catalysts, i.e., Ziegler-Natta catalysts.
Along with the better known EP and EPDM polymers, co- and terpolymers incorporating other .alpha.-olefins in place of propylene such as 1-butene, 1-pentene, 1-hexene, styrene, and combinations thereof are also known. EPDMs are representative of the more general category of ethylene-.alpha.-olefin diene elastomers (EODEs). Of the EODEs, EPDMs have achieved particular prominence due to the many properties which make them desirable for applications requiring good weather and acid resistance and high and low temperature performance. Notable applications of the EPDMs include their use in such products as hoses, gaskets, power transmission belts, conveyor belts, bumpers, automotive extrusions and moldings, weather stripping, blending components for plastics and rubbers such as polypropylene, polystyrene and butyl rubber, fabric coatings, viscosity modifiers for lubrication oils, tire sidewalls and in roofing and other membrane applications, shoe soles and heels and many other rubber articles. Another noteworthy application of the EPDMs is in wire and cable insulation due to their excellent dielectric properties.
It is desirable for an EPDM to have a reasonably fast cure rate and high state of cure, requirements calling for a relatively high diene content, e.g., three percent or higher. The cure rate for an EPDM elastomer and the final properties of the cured article depend upon the type of diene incorporated. For example, on a comparable diene weight percent basis, an EPDM produced with 5-ethylidiene-2-norbornene (ENB) as the diene will have a faster cure rate using a sulfur cure than would an EPDM containing dicyclopentadiene (DCPD) or 1,4-hexadiene (HD).
As for the properties of cured EPDM, EPDMs made with hexadiene as the termonomer are known to exhibit good heat resistance. For most commercial elastomer applications, the EPDM should have a weight-average molecular weight (M.sub.w) of at least about 300,000, or ML.sub.1+4 at 125.degree. C. of at least about 20 when expressed in terms of Mooney viscosity. In many applications, it is further desirable that the molecular weight distribution (MWD) of an EPDM be characterized by a ratio of weight average molecular weight to number average molecular weight (M.sub.w /M.sub.n), i.e., polydispersity index, of not greater than about 7 and preferably not greater than about 5.
The properties of an EPDM elastomer such as its tensile strength, processability and tack can be related to its degree of crystallinity. Since in most commercial uses elastomers are higher in molecular weight than plastics, too high a degree of crystallinity can make an EPDM difficult to process at ordinary temperatures. Although good physical properties are desirable, especially in such applications as hose, tubing, wire and cable, excessive crystallinity can cause an EPDM to exhibit high hardness and stiffness resulting in a "plastic" rather than a "rubber" surface with poor surface tack.
In general, commercially useful plastics, which are homo- and copolymers of ethylene, propylene, and higher .alpha.-olefins, need not have as high a molecular weight as commercially useful elastomers of ethylene-.alpha.-olefins such as EPDM. In terms of the catalysts used for each, when producing copolymers with compositions of M.sub.w in the elastomer range, catalysts that provide high M.sub.w plastic copolymers may produce low M.sub.w polymers unsuitable for elastomer applications. Similarly, undesirable MWD changes can occur or the compositional distribution can change. Thus, catalyst performance for the production of plastics is not indicative of catalyst performance for the production of elastomers.
In most current EPDM production, the catalysts conventionally employed in the production of high molecular weight EPDM elastomers are soluble vanadium catalysts such as VCl.sub.4, VOCl.sub.3, VO(Ac).sub.3 or VO(OR).sub.3 where R is an alkyl group together with an organoaluminum compound. The activity of the vanadium catalysts are relatively low, e.g., producing 5-20 kg polymer/g vanadium.
In current commercial grades of EPDM, crystallinity is a function of both the ethylene content of the polymer and the catalyst system used for its production. For a given polymer composition, the catalyst system controls the fraction of ethylene units present in long ethylene sequences which are capable of crystallizing. With any given catalyst and reactor configuration, polymers with higher ethylene content will have longer ethylene sequences and be more crystalline.
In current EPDM production based on vanadium catalysts, the product EPDM polymers are completely amorphous (non-crystalline) at ethylene contents below about 55 wt %. Conversely, at ethylene contents of about 55 wt % or greater, an EPDM will possess significant crystallinity. The degree of crystallinity depends less on the diene content of the EPDM than on the percentage of ethylene.
In order for the catalyst system to be useful for the commercial production of an EPDM elastomer, it is desirable for the crystallinity of the polymer to be roughly comparable to that of currently available commercial grades of EPDM for most applications.
Metallocene catalysts typically consist of a transition-metal atom sandwiched between ring structures to form a sterically hindered site. Plastics obtained with metallocene catalysts tend to have increased impact strength and toughness, good melt characteristics, and improved clarity in films.
In actual practice, the extent to which metallocene catalysts can effectively replace traditional catalysts in polymer production depends on the cost and efficiency of the system. Metallocene catalysts cost significantly more than the traditional Ziegler-Natta catalysts but the metallocene systems are considerably more productive. In some cases, the increased productivity of metallocene catalysts relative to the Ziegler-Natta catalysts ranges from one to two orders of magnitude more polymer produced per pound of catalyst.
An example of the use of metallocene catalysts in polymer production is in U.S. Pat. No. 5,304,614 which discloses a process for polymerizing or copolymerizing an olefin in the presence of a catalyst. The catalyst employed is formed from a metallocene procatalyst that has been activated by an aluminoxane and/or compounds of the general formulae R.sub.x NH.sub.4-x, BR'.sub.4, R.sub.x PH.sub.4-x BR'.sub.4, R.sub.3 CBR'.sub.4 or BR'.sub.3 where x is a number from 1 to 4 and R can be equal or different and is a C.sub.1 -C.sub.10 alkyl or C.sub.6 -C.sub.18 aryl which can be substituted by an alkyl, haloalkyl or fluorine.
Since the recent introduction of the aluminoxane-activated metallocene catalysts for producing polyethylene, polypropylene, and copolymers of ethylene and .alpha.-olefins such as linear low density polyethylene (LLDPE), efforts have been made to apply these catalysts to the production of EPDM elastomers. For this use, it is desired that the catalyst produce high yields of EPDM in a reasonable polymerization time, result in adequate incorporation of the diene monomer(s) and provide a random distribution of monomers while enabling good control of M.sub.w over a wide range while yielding a relatively narrow MWD. However, one of the obstacles to widespread commercial implementation of metallocene catalysts lies in the use of an aluminoxane as cocatalyst. Aluminoxanes are expensive and large amounts are required in order to activate the metallocene catalyst with which they are associated.